This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Medimmune, Inc. has an FAB which tightly binds to a specific viral F protein, which acts as an antiviral agent. While the FAB has been biochemically characterized, structurally, it remains unclear exactly how the binding occurs. This knowledge would be valuable to Medimmune in development of future similar products. Medimmune is providing F protein trimers with bound antibodies in aqueous buffer for cryo-EM characterization. We attempt to solve the structure at the highest resolution possible, hopefully to subnanometer resolution. Medimmune has awarded Dr. Ludtke a small grant to study this interaction in collaboration with Dr. Oganesyan.